Published in 1967 and then extensively revised in 1978, The Ninth Configuration is a fascinating look at madness, philosophy, and the nature of religious belief. William Peter Blatty is most well-known for his novel The Exorcist, and his Academy-award winning screenplay for the movie. He also wrote a sequel to The Exorcist, Legion, and directed Exorcist III as well as the film version of The Ninth Configuration. Mark Kermode is a well-known film scholar and expert on William Peter Blatty's fiction and films.
When originally published in 1971, The Exorcist became not only a bestselling literary phenomenon, but one of the most frightening and controversial novels ever written. (When the author adapted his book to the screen two years later, it then became one of the most terrifying movies ever made.) Blatty fictionalized the alledgeded true story of a child's demonic possession in the 1940s. Of course, it is still jus a book :)